McGregor will make his professional boxing debut against Mayweather at the T-Mobile Arena in Las Vegas on Aug. 26 in a fight predicted to make millions.

But some boxing fans -- including former undisputed world heavyweight champion Lewis -- will not be buying the fight on pay-per-view.

"I can't take it serious," Lewis told ESPN. "Mayweather is the best in his weight class; no one can touch him in boxing. He's a pugilist of the highest order, so for another man from a different sport to fight him?

"It's ridiculous to me. You are talking about one discipline where you can use only your hands and only a few can do it. Every battle sport always starts with your hands, but they are two totally different sports.

"Floyd will stop him. He's just too good at boxing for McGregor, and McGregor is just too inexperienced. I wouldn't buy it, but I would be interested in the result. To me, I know who is going to win already, so there's no use in watching it."